The key witness in the case against Scott Watson was pressured and manipulated by police to identify him as the man last seen with murder victims Ben Smart and Olivia Hope, the Court of Appeal has been told.
Watson is attempting for the fourth time to overturn his convictions for the murders on New Year's Day in 1998 in the Marlborough Sounds.
In summing up the defence's case on Wednesday, lawyer Nick Chisnall KC said the Crown's case completely relied on the positive identification of Scott by water taxi driver Guy Wallace, who dropped off the young pair to a stranger's yacht in the early hours.
It was now known that after initially describing a completely different man and failing to pick Watson out of a montage of photos, Wallace was shown a single picture of Watson.
Chisnall said this was an investigative strategy, "rather than a misstep by an overzealous officer failing to follow instructions".
Three months later, after being shown so-called 'montage B' - which Chisnall said was carefully crafted by senior detectives to more closely match Wallace's original description - he picked out Watson as the man he had seen.
Wallace - who has since died - recanted soon after, saying he was pressured by police.
That positive identification was critically important to the Crown case, as the Court of Appeal had confirmed in 2000 and again in 2022.
However, after 25 years, this was the first time the court had been asked to examine "the manifold defects in the procedures adopted by police to secure the evidence", Chisnall told the court.
It was clear that police exerted "significant interrogative and moral pressure" on Wallace to change his story.
Furthermore, the importance of that single photo - and possible contamination effect - was never made clear to the jury at trial (Watson's original lawyers said they were never made aware of it, although the Crown disputes that) and the judge's final direction to the jury also glossed over it, Chisnall said.
The Crown was putting much emphasis on "freshness" when it came to facts, implying that all this evidence was available at the original trial and put before the jury. However, Chisnall said that was not the case.
"There are and perhaps always will be cases in which concerns entertained by convicted person's whanau or defence team over the justice of the conviction persists, even after all orthodox avenues of review have been exhausted.
"Such concern is not just for the convicted person and their family, but the integrity of the system itself."
The criminal justice system must be "willing to accept the possibility that mistakes leading to injustices may be made", he said.
"It's an important matter, not just for Watson and Hope's family and Smart's family, but also for the integrity of the criminal justice system."
